Seat belt systems were developed to secure the occupant of a vehicle against harmful movement that may result during a collision or a sudden stop. The systems reduce the likelihood and severity of injury in a traffic collision by stopping the vehicle occupant from hitting hard against interior elements of the vehicle or other passengers (the so-called second impact), by keeping occupants positioned correctly for maximum benefit from the airbag and by preventing occupants being ejected from the seat of the vehicle.
A traditional seat belt system usually comprises a seat belt for restraining the occupant in the seat, a belt winder for tightening or loosing the seat belt, a tongue plate slidably arranged on the seat belt, and a belt buckle assembly which comprises a belt buckle detachably engagable with the tongue plate, and a driver for moving the belt buckle relative to the occupant. The movement of the belt buckle is able to adjust the effective length of the seat belt, so the belt buckle assembly is typically used as a belt tensioner.
FIG. 4 illustrates a winding scheme of a traditional permanent magnet direct current (PMDC) motor used for driving the belt buckle in a seat belt system. The rotor windings comprise a plurality of coils wound about teeth of the rotor core and electrically connected to segments of a commutator. For example, coil C1′ is wound about teeth T1 and T2 and connected to segments S1 and S2. Coil C2′ is wound about teeth T2 and T3 and connected to segments S2 and S3. The PMDC motor comprises two brushes to feed electric current to the rotor windings. The rotor windings form two parallel branches. When a coil such as coil C1′ is open, one of the branches will be open. The motor as well as the seat belt system will malfunction. It is dangerous to drive a car having a malfunctioning seat belt buckle assembly.
Therefore, there is a desire for a seat belt buckle assembly with improved reliability.